Spectrum analyzers that are currently available, such as the Tektronix 49X series of spectrum analyzers, provide a display in a rectangular Cartesian coordinate system of values of signal power plotted along a vertical axis and values of signal frequency plotted along a horizontal axis. In a typical spectrum analyzer, the display has 1,000 resolution points in the horizontal direction, each representing a frequency band, and 225 resolution points in the vertical direction, each representing a power band, for a total of 225,000 addressable display points. In the event that the display is provided on the screen of a cathode ray tube (CRT), the display is created by sweeping the electron beam across the screen of the CRT in a succession of vectors from one addressed display point to the next in the direction of increasing frequency.
Current spectrum analyzers have a real time mode of operation and a storage mode. In the storage mode, as few as 11 or as many as 22,000 sample values are generated for each of the horizontal resolution points. Each sample value represents the value of signal power at a frequency in the frequency band represented by the particular horizontal resolution point. However, such spectrum analyzers are limited to providing a display of only a single power value for a given frequency value. Therefore, in the storage mode the multiple power values are processed to yield a single value, e.g. the peak value or the mean value, and this single value is stored and is used to provide the display. Accordingly, the spectrum analyzer's memory has 1,000 memory locations, each of which is capable of storing at least 225 distinct sample values. The display that is provided by a spectrum analyzer operating in the storage mode is stable but does not convey information regarding the distribution of the power levels in a particular frequency band.
In the real-time mode, the display point currently being addressed represents the real-time value of signal power at a particular frequency. For a given frequency band, several power values may be displayed. The intensity of a particular display point depends upon the amount of time for which the input signal has the combination of power and frequency values represented by that point. If the input signal is time-varying, the display that is provided in the real-time mode conveys useful information regarding the distribution of power values in a particular frequency band. However, if the variations in signal power over time are substantial, the display may be disturbing to the user because it is not stable. Also, if the display refresh rate is low, the display is objectionable because it flickers in intensity.